1. Field of the Invention
The present invention generally relates to forming methods of electrodes and manufacturing methods of semiconductor devices. More specifically, the present invention relates to a forming method of an electrode having a reflow process using a reducing atmosphere and a manufacturing method of a semiconductor device having the electrode.
2. Description of the Related Art
A projection electrode made of spherical-shaped solder has been applied as an outside connection electrode of a semiconductor element such as a semiconductor integrated circuit element or an outside connection terminal of a wiring board where the semiconductor element is mounted.
Such a projection electrode is called a bump. In a case where the projection electrode is made of solder, lead (Pb)—tin (Sn) group solder or so-called lead free solder such as zinc (Sn)—silver (Ag) instead of lead group solder has been used. The solder is metal having a low melting point. An electrolytic plating method, an electroless plating method, a paste transferring method or a printing method is used as a method for forming the solder on the semiconductor element.
Since the solder formed by the above-mentioned method has a plane plate-shaped configuration, a columnar-shaped configuration or a mushroom-shaped configuration, a reflow process is applied after the solder is formed so that a solder bump having a substantially spherical-shaped external configuration is formed.
A method for connecting the semiconductor element on an electrode terminal on the wiring board using a solder bump (conventional solder or lead free solder) formed on a main surface of the semiconductor element is called a flip chip method or a face-down method because a main surface (electric circuit forming surface) of the semiconductor element faces a surface of the wiring board. In a case where the semiconductor element is mounted on the wiring board by such a flip chip method (face-down method), the solder bump is melted (reflows) and an oxide film on a surface of a terminal of the wiring board is removed by using flux so that the surface of the terminal is cleaned.
In a step of connecting solder using the flux, it is necessary to remove solidified flux with an organic solvent after the solder bump is connected.
However, the solidified flux cannot be sufficiently dissolved by a small amount of the organic solvent and a product material generated by dissolution cannot be completely removed. Because of this, the amount of the organic solvent used may be increasing. Use of a large amount of such an organic solvent may have an adverse impact on environment.
On the other hand, as the semiconductor elements have higher integration and are made minute, there is tendency that a size (height) of the solder bump is made small (short). Hence, a gap between the semiconductor element and the circuit board is narrower.
In addition, since high functionality and multiple functions are required for the semiconductor elements, there is tendency that the external dimensions of the semiconductor elements become larger so that an area facing the wiring board increases. Accordingly, it becomes difficult to implement a cleaning process using the organic solvent.
Because of this, recently, a reflow processing method has been suggested where hydrogen (H2) and formic (methanoic) acid (HCOOH) are used as a reducer and the cleaning process is not required.
For example, a method for implementing a reflow process in a chamber having a reducing gas environment made of mainly hydrogen (H2) has been suggested as a method for reflowing cream solder in a reducing gas environment. See Japanese Patent Application Laid-Open Publication No. 6-226437.
In addition, the following method has been suggested as a manufacturing method of a semiconductor device having a solder reflow step using formic (methanoic) acid (HCOOH) as a reducer. That is, first, a solder layer is formed on a metal film of the semiconductor device. The semiconductor device and the solder layer are arranged in a depressed environment including the formic acid. The solder layer is heated and melted in the environment. After that, the environment is discharged and the solder layer is held at a temperature lower than the melting point of the solder and equal to or higher than the boiling point of formic (methanoic) acid (HCOOH) for a designated time. See Japanese Patent Application Laid-Open Publication No. 2001-244283.